docs: Update FAQ regarding ZSTD and CGO (#1766)
docs: Update FAQ regarding ZSTD and CGO (#1766)

- the library used for ZSTD was replaced by a pure Golang version
  so CGO is no longer required. Update the FAQ to reflect this
- link to the discussion regarding backwards compatibility
  so it's easy to find.
dgraph-io/badger,dgraph-io/badger